The present invention relates to a method of determining the perimeter of the visual field of the human eye.
The visual field of the human eye is determined by the retinal light-sensitivity. The light-sensitivity values within the visual field are determined by means of a perimeter. For this purpose, various methods are known, in all of which a number of test sites within the visual field are examined and for which the threshold value of perception is determined. Computer-controlled perimeters are suitable, which are programmed in accordance with the method of determination.
It is known to determine the retinal light-sensitivity at the specific test sites stipulated by coordinates by applying light stimuli of different intensity (brightness or magnitude) using a perimeter. The subjects respond during an examination by pressing a response button when the light stimulus is perceived. If the light stimulus is not seen, the response button is not pressed. During this procedure, the light stimuli are applied in a graduated manner with greater and lesser intensity in such way that the sensitivity threshold is exceeded one or more times. After the sensitivity threshold has been exceeded, the direction of alteration of the stimulus intensity changes in each case from brighter to darker or vice versa. At the same time, the magnitude of the alteration is reduced (e.g. to half the previous value) until the sensitivity threshold has been bracketed, that is to say the threshold value of perception has been determined.
By definition, the threshold value of perception (retinal sensitivity in decibels) corresponds to that stimulus intensity (in apostilb) of which, after a specific number of applications, half were seen and half were not seen. The retinal sensitivity is expressed as a logarithmic function of the quotient calculated from the maximum stimulus intensity divided by the stimulus intensity applied. The formula for retinal sensitivity is: ##EQU1##
This means that the retinal sensitivity is identified as 0 dB if the brightest stimulus (e.g. 1,000 or 10,000 asb) is not seen. High stimulus intensities correspond to low sensitivity values.
According to this known method, determining the retinal sensitivity by bracketing requires four to six stimulus applications per test site. Since the retinal sensitivity of the visual field is usually determined by measurements at 50 to 100 test sites, between 200 and 600 stimulus applications are required for each examination of the visual field. Such an examination therefore usually lasts between 7 and 20 minutes. For reasons of efficiency, due to accuracy problems caused by the progressive slowing-down of retinal renewal (fatigue) and due to insufficient reliability through incorrect responses with diminishing concentration of the subjects, this duration of the examination is considered to be too long.